To make the motor vehicle easier to drive and avoid collisions with cars or other objects in the way, it is known to provide sensors at the front and/or rear of the motor vehicle, these sensors transmitting ultrasonic or radar signals, for example, and receiving the signals reflected from the obstacle. In this arrangement, the distance between the sensor arranged on the motor vehicle and the obstacle is determined from the propagation time of the signal from the sensor to the obstacle and back again.
A collision warning system of the same generic type is known from the as yet unpublished patent application DE 197 11 467.9, in which contactless distance measurement between an obstacle and a motor vehicle is carried out with a plurality of sensors. In this arrangement, one sensor transmits a sensor signal in all cases, and this signal is reflected by an object situated in the sensor beam. The reflected signal is received by all the sensors. To enable imaginary objects reliably to be excluded, all possible positions of the object relative to the motor vehicle are determined from the propagation times of three sensors. Only those positions of the object are recognized as real which are detected by all three sensors.
Particularly for propagation-time measurement, referred to as indirect, in which the signal is transmitted by one sensor and the reflected signal is received by another sensor, it is necessary that the sensors should be synchronized with the aid of an evaluation device, ensuring that each sensor is ready to receive when the signal reflected by the obstacle reaches it. Here, the evaluation device controls the time sequence in such a way that one sensor transmits a signal and all the sensors are then simultaneously switched to the receptive condition.
Particularly when using radar sensors, the electrical synchronization of the sensors is very complex, if not impossible, due to the rapid signal propagation times of the transmitted and reflected sensor signal, since the propagation times of the control signals on the connecting lines between the evaluation device and the sensor are always slower than the sensor signals, which propagate approximately at the speed of light.
Thus, there is a need to specify an arrangement for contactless measurement of the distance between an object and a motor vehicle, in which the outlay for electrically synchronizing the sensors is reduced and which nevertheless allows reliable detection of the actual object for accurate measurement of the distance.
Other needs will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings.